The operation mode of an unmanned aerial vehicle (UAV) mainly includes a manual flight mode and an automatic flight mode. In the automatic flight mode, an obstacle avoiding method must be provided so that the unmanned aerial vehicle can avoid obstacles and arrive at the destination safely. The obstacle avoiding method normally needs to scan the environment or provide various environmental parameters (such as geographical information) to create an environment model so as to determine a flight path with reference to the environment model to avoid the obstacles. However, it requires a large amount of time to create and maintain an environment model. Additionally, the calculation of flight path according to the environment model requires a large volume of data computation. To calculate the flight path, the unmanned aerial vehicle must be equipped with high-efficiency processor, which incurs extra cost.